Or at least add an asterisk that points to a note saying "in a few decades, maybe."

Plug-in electric cars have many appealing features: they're nicer to drive, they're cheaper per mile to operate than the average conventional car in almost all U.S. locations, and they are partially or fully zero-emission.

They are still notably pricier to buy than conventional cars of the same size, however, though the steady 7-percent annual cost reduction in lithium-ion cell prices will steadily chew away at that problem until it's eliminated.

Even with global sales approaching 100 million new vehicles a year, making a significant dent in the global fleet takes many, many years.

Consider the case of catalytic converters to reduce emissions of carbon monoxide, hydrocarbons, and nitrogen oxides.

catalytic converter

They were first introduced in the U.S. on 1975 model-year cars. They didn't become common in Europe for another 10 years, and the rest of the world followed in the 1990s and 2000s.

In other words, it took 30 years for the technology to become standard—and far longer for it to spread into a majority of those 1.2 billion vehicles.

(3) An entirely new and costly industry has to be built

That 7-percent annual cost reduction metric for lithium-ion cells is based on more than 20 years of data from consumer-format cells.

But to move even a substantial minority of the world's 100 million vehicles to partial or full electric power will require an entirely new network of global lithium-ion cell production.

Oil well (photo by John Hill)

Remember, a mobile phone has one lithium-ion cell; a laptop may have four to eight. A Tesla uses between 5,000 and 10,000 of them.

That's why Tesla Motors CEO Elon Musk, several years ago, identified the need for the company to build a "gigafactory" that would produce cells and battery packs at a scale that hadn't been seen anywhere in the world.

To put large battery packs, with ranges of 50 miles (for plug-in hybrids) to 300 miles (for battery-electrics), into even one-fifth of those 100 million vehicles will require hundreds or thousand of gigafactories.

With a cost of $2 billion to $5 billion, that ain't chump change.

It will come, if the demand is there, but automakers are understandably cautious about signing contracts for cell purchases for vehicles whose demand thus far remains uncertain.

A "hockey-stick" rise in consumer demand for plug-in electric cars may not arise for several more years.

Again, it will require the cars to cost within 10 or 20 percent of a comparable vehicle with a gasoline engine alone.

Gas prices seem to matter—they're currently low, and projected to stay that way for some time, absent geopolitical surprises—as do the hurdles posed by franchised auto dealerships and inept marketing by major automakers.

There's no indication, however, that lithium-ion cell costs won't continue to fall.

So when cells fall below $100 per kilowatt-hour, meaning a 60-kwh battery pack would cost $5,000 or less, the picture begins to change.